Generally, window film provides specific personal and property protection from the effects of the sun as well as added safety and security in the events that result in broken glass. The concept of window film for use in solar control flat glass application dates back to the early 1960s. The original film design objective was to control the heating and cooling imbalances that result from solar loading. Such early films were found to reflect solar radiation back from a window, preventing the warming of inside surfaces normally hit by direct sunlight while still allowing vision through the glass. As the window film concept was developed and improved upon, a demand developed for colored sun control films that would complement architectural design. Coloration of film was achieved through various means to produce colors such as bronze, gray, gold, amber, etc. The energy crisis of the early 1970s prompted an interest in another aspect of window film use: the reduction of heat loss to the outside. It was discovered that polyester film tended to absorb and reradiate long wave infrared heat rather than act as a transparent medium. Through experimentation, new film materials and constructions were developed that enhanced this characteristic. These films greatly improved heat retention within a room's interior. The efficiencies of solar control window films are closely related to local weather conditions, building orientation, window size, and other factors such as exterior shading conditions. However, with escalating energy costs, products such as window film are increasingly valuable as an investment for commercial and residential owners and commercial facility managers. There are many types and constructions of solar control and safety window films. These films are considered in the building industry to be "retrofit" products; that is, products to be applied to existing buildings as opposed to use in new construction. In their simplest forms, window films are composed of a polyester substrate to which a scratch resistant coating is applied on one side; a mounting adhesive layer and a protective release liner is applied to the other side. When the release liner is removed, that side of the film with the adhesive is applied to the interior surface of the glass.

Definition of Terms

Total Solar Transmittance - The ratio of the amount of total solar energy in the full solar wavelength range (300-2,100 nanometers) that is allowed to pass through a glazing system to the amount of total solar energy falling on that glazing system. Value is usually expressed as a percent.

Total Solar Reflectance - The ratio of total solar energy which is reflected outward by the glazing system to the amount of total solar energy falling on the glazing system. On filmed windows this reflectance is a function of the side of the film facing the window surface. Value is usually expressed as a percent.

Total Solar Absorption - The ratio of the amount of total solar energy absorbed by a glazing system to the amount of total solar energy falling on the glazing system. Solar absorption is that portion of total solar energy neither transmitted nor reflected. Since solar transmittance and solar reflectance are measured directly, the following equation should be used in calculating solar absorption. Solar absorption =1.00 - (solar transmittance) - (solar reflectance)

Visible Light Transmittance - The ratio of the amount of total visible solar energy (380-780 nanometers) that is allowed to pass through a glazing system to the amount of total visible solar energy falling on the glazing system. Value is usually expressed as a percent. Glare is influenced by visible light transmittance through a glazing system.

Visible Light Reflectance - The percent of total visible light to be reflected by a glazing system that can be seen visually.

Ultraviolet (UV) Transmittance - The ratio of the amount of total UV solar energy (300-380 nanometers) that is allowed to pass through a glazing system to the amount of total UV solar energy falling on the glazing system. Ultra-violet is one portion of the total solar energy spectrum which greatly contributes to fading and deterioration of fabric and furnishings.

U-Factor - The overall heat transfer coefficient of the glazing system, U -Factor is a measure of the heat transfer that occurs through the glazing system, and its outer and inner surfaces. This value is a function of temperature, and is expressed in BTU per square foot per hour per degree Fahrenheit (BTU/ sq. ft./hr. degrees F). The lower the U- Factor, the better the insulation qualities of the glazing system.

Shading Coefficient - (SC) The ratio of the solar heat gain through a given glazing system to the solar heat gain under the same conditions for clear, unshaded double strength window glass (DSA). Shading coefficient defines the sun control capability or efficiency of the glazing system.

Total Solar Energy Rejected - The percent of incident solar energy rejected by a glazing system equals solar reflectance plus the part of solar absorption which is re-radiated outward.

BTU-British Thermal Unit - The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. 1 BTU = 252 Calories (CAL) TON-HR - One ton of air conditioning = 12,000 BTU/Hour (3.52kW)